This invention relates generally to electromagnetic flowmeters, and in particular to a unitized electrode assembly for such flowmeters.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field normal to the direction of flow being established by an electromagnet. When the flowing liquid intersects this field, a signal is induced therein which is transferred to the detecting electrodes. This signal, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator. The magnetic field may be either direct or alternating, for in either event the amplitude of signal induced in the liquid passing through the field will be a function of its flow rate.
In a flowmeter of this type disclosed in the German patent publication DT-AS 1,963,901, the flow tube is lined with dielectric material and is provided with at least two planar electrodes having a relatively large area, the electrodes being in electrically-conductive contact with the liquid passing through the tube.
The main concern of the present invention is with flowmeters of the type disclosed in German Patent No. 1,473,041 and in U.S. Pat. No. 3,274,831, wherein a flowmeter energized by alternating-current field is provided with two or more planar detector electrodes of large area and a shielding electrode of even larger area cooperating with each detector electrode. For measuring dielectric fluids, the electrodes are covered by the dielectric lining of the flow tube and are thereby protected from the fluid. This lining, together with the dielectric of the fluid, constitutes the dielectric of a capacitor whose plates are formed by the planar measuring electrodes. A similar arrangement is disclosed in the pending U.S. application Ser. No. 617,982, filed Sept. 29, 1975, whose entire disclosure is incorporated herein by reference.
In the arrangement disclosed in U.S. Pat. No. 3,274,831 and in German Patent No. 1,473,041, the dielectric liner for the flow tube is constituted by two distinct cylindrical bodies, one being received within the other. The faces of the detecting electrodes lie against the outer surface of the first dielectric cylinder. The rear and sides of these electrodes adjoin the second dielectric cylinder, the shielding electrodes being embedded in the second cylinder. Connections to these electrodes are made by way of a coaxial line through an opening in the flow tube and through an opening in the second dielectric cylinder of the liner, the coaxial line leading to external detector amplifiers.
Experience has shown that in an arrangement of this known type, because of the extremely high impedance of the capacitively-coupled detecting electrodes, pressure and temperature changes, flow tube vibrations and other environmental influences give rise to a considerable degree of error which is not compensable.
It has been found that a major cause of such error or signal distortion are microphonic effects which produce crevices, fine cracks or fissures between the electrode surfaces and the cylinders of the dielectric liner, or which result in slight motion of the electrode connections with respect to the magnetic or electric fields of the exciter circuit of the flowmeter. These motions create spurious signals which render accurate flow rate measurement virtually impossible.